Description of the Prior Arts
The cold isostatic press method, hereafter abbreviated to CIP method, has been customarily used for pressure forming or molding. According to this method, metallic or ceramic powders are charged into a pouch of rubber-like resilient material which is then hermetically sealed and pressured from outside by a liquid such as water or oil as pressure medium to effect pressure forming or molding.
In this case, a rubber-like mold, hereafter abbreviated to rubber mold, usually formed of rubber, PVC or latex such as polyurethane, is used.
It goes without saying that the rubber mold should be of a strength and a thickness sufficient to prevent the mold from being deformed under the weight of charged powders.
In carrying out the aforementioned method, because of the different behavior in deformation between the rubber and the charged powders, it is a frequent occurrence that the hydrostatic pressure applied from outside the rubber mold is not directly transmitted to the charged powders and construction of the powders at the corner area is inhibited by the rubber material.
Therefore, the molded body not only tends to be deviated in shape from the rubber mold cavity under no-loaded conditions, but also tends to be cracked under the effect of the residual inner stress.
Hence, difficulties are presented in the conventional CIP method in obtaining an impeccable molded product having a high dimensional accuracy.
The inventors conducted eager researches into solving the aforementioned problem and arrived at an improved CIP method which constitutes the subject-matter of the Japanese Patent Application No. 59-183780 corresponding to U.S. Pat. No. 4,612,163.
In these applications, there is described a method for forming a mold while a tension is applied to the thin-walled rubber-like material. According to this method, since the rubber-like pouch is contracted with contraction of the charged powders, these powders are contracted uniformly, thus resulting in a molded body analogous in form to the initial charged material.
In more detail, to a gate member of an air permeable porous mold carrier is intimately secured the mouth of a thin-walled rubber-like pouch and the air outside of the air permeable mold carrier is exhausted for expanding the ru ber-like pouch into intimate contact with the inside of the mold carrier for forming the mold.
Then the starting powdered material is charged into the thus created mold space and the opening of the mold is sealed after the air is exhausted from the inside of the mold.
The atmosphere outside the air permeable mold cavity is reset to the atmospheric pressure to disintegrate the mold for taking out the pre-molded body which is processed with CIP for improving its density.
It is stated in the aforementioned applications that molded products of porous ceramics of polyamide resin, porous sintered alloy, porous ceramic-alloy composite material or plaster are preferred as air permeable mold carrier material.
However, these air permeable mold carriers tend to be costly since the molded products have to be produced with sufficient dimensional accuracy of the mold cavity and sufficient surface properties to permit shipping of the rubber-like resilient material.
In this manner, the method can be applied only to cases wherein a larger output can be expected from the molding operation.